This invention relates to diaphragm gas meters and, more particularly, to an improved tangent assembly for use therewith wherein a cam is utilized to adjust the stroke of the meter diaphragms.
Positive displacement diaphragm gas meters typically are provided with a tangent assembly which drivingly couples the diaphragm driven flag assembly to the crank shaft, which is in turn coupled to open and close the slide valves of the meter. It is the motion of the flag assembly, along with the opening and closing of the valves, which determines how much gas is displaced per stroke of the diaphragms. The motion of the flag assembly is typically determined by a linkage system that is centered upon a common wrist. This wrist is supported on a tangent assembly which connects to a crank that controls the opening and closing of the slide valves. It is common practice to provide a tangent assembly which is adjustable n order to change the position of the wrist relative to the crank, thereby affecting the xe2x80x9cstrokexe2x80x9d of the diaphragms as well as the xe2x80x9ctimingxe2x80x9d of the valves.
The stroke is determined by the radial distance from the wrist to the center of rotation of the crank. The greater this distance, the larger the volume of gas displaced per revolution of the tangent assembly. The timing of the valves is determined by the angular position of the wrist around the crank. It is desirable when adjusting one of the stroke and timing parameters that the other parameter is not affected. It is also desirable that the parameters are easy and quick to adjust, in order to lessen the calibration time. Ideally, it would be desirable to simplify the adjustment to the point where an automatic calibration system can be provided to eliminate the need for human operator intervention.
According to the present invention, there is provided an improved tangent assembly for a positive displacement diaphragm gas meter. The inventive tangent assembly drivingly couples a flag assembly to a crank shaft rotatable about an axis and includes a base non-rotatably connected to the crank shaft along the axis of rotation of the crank shaft and a plate overlying the base and slidable thereon. The base is formed with a circular opening concentric with the crank shaft axis of rotation. The plate is formed with a stepped opening therethrough which is alignable over the circular opening of the base. The stepped opening has a lower opening in the shape of an elongated slot with a width greater than the diameter of the circular opening and an upper opening of larger dimension than the lower opening. The upper opening has a pair of opposed cam follower projections extending therein along a central major longitudinal axis of the elongated slot. A wrist pin is fixedly connected to the plate and extends outwardly therefrom parallel to the crank shaft axis rotation, and the flag assembly is connected to the wrist pin. A stroke adjuster is provided for slidably moving the plate relative to the base so as to vary the distance between the wrist pin and the crank shaft axis of rotation. The stroke adjuster includes a pointer overlying the plate and a post secured to the pointer and extending through the stepped opening of the plate and into the circular opening of the base. The stroke adjuster post has first, second and third levels. The first level is adjacent to the pointer and includes a cam rotatable within the upper opening of the plate and engaging both of the pair of cam follower projections. The second level of the post is adjacent to the first level and is cylindrical with a diameter equal to the width of the elongated slot of the lower opening of the plate and is rotatable in the elongated slot. The third level of the post is adjacent to the second level and is cylindrical with a diameter equal to the diameter of the circular-opening of the base and is rotatable in the circular opening. Accordingly, rotation of the stroke adjuster results in a change of the distance between the wrist pin and the axis of rotation of the crank shaft.
In accordance with an aspect of this invention, the profile of the cam results in a linear relationship between angular movement of the stroke adjuster and change of distance between the wrist pin and the crank shaft axis of rotation.